Xephem is an interactive astronomical ephemeris program for the X Window
system. It ...

    computes heliocentric, geocentric and topocentric information for fixed
    celestial objects and objects in heliocentric and geocentric orbits;

    has built-in support for all planet positions; the moons of Jupiter, Saturn
    and Earth; Mars' and Jupiter's central meridian longitude; Saturn's rings;
    and Jupiter's Great Red Spot
